1. Technical Field of the Invention
The present invention relates to generally to a method of detecting and identifying the most beneficial point to part or cut well pipe in order to recover it from a well. More specifically, the present invention relates to a method and apparatus to determine the location of the point along a length of well pipe where the well pipe is bound by rock, mud, or cement.
2. Description of Related Art
The ability to locate the point at which a tubular is stuck within another or within a well bore is useful. An accurate determination of the location of a stuck point (also referred to as a “freepoint”) makes it possible to position tools to conduct recovery operations. Prior art devices include a number of devices which are intended for down-hole deployment. Most of these tools require applying tension or torsion to the well pipe. By measuring certain characteristics before application of the force and during application of the force, a determination can be made regarding the location of the sticking point.
Such known devices typically fall into two general categories. One category of tools measures well pipe displacement when stress is introduced into the well pipe. For example, the well pipe may be stretched or twisted and physical distance measurements quantify the movement or displacement of the well pipe or a section of the well pipe when it is stretched or twisted. These measurements are used to calculate how much of the well pipe is above the freepoint. A second type of tools relies on the ability to detect changes in a well pipe characteristic other than displacement. Various such detection methods include Hall Effect devices, strain gauges, and devices measuring magnetic permeability.
An example of such a device is disclosed in U.S. Pat. No. 4,708,204. The device disclosed in U.S. Pat. No. 4,708,204 detects changes of magnetic permeability when a motive force, such as tension or torque, is applied to a well pipe. Another known device is disclosed in U.S. Pat. No. 4,766,764, which discloses a device that uses Hall Effect sensors to measure and compare the absolute magnetic strength in the well pipe.